Supergirl #1

I just started reading Crisis on infinte earths....This has lil bit of a tie in... well, all of the DCU has spun off from that story arc.....

I was always confused and how and who are supergirl and power girl related,, ... I think there the same person...one from the future ... but the question is how?? I mean they both have very different body types,, supergirl , is a skinny 16 year old,, power girl, well, uh,,shes different.. But they had a weird reaction when they met,,hmmm I want to see how this turns out,, also with the JSA Classified, where power girl goes on her quest to find her true origins..